This invention relates to crystal seed holder assemblies and, more particularly, to such assemblies used to grow a ribbon of crystal from a liquid melt by EFG (Edge-defined Film-fed Growth) techniques. When thus growing ribbon, a crystal seed is initially placed adjacent the growth control surfaces of a capillary passage in a conventional die member. By maintaining appropriate temperature and other conditions the liquid melt in the capillary will solidify on the seed; by pulling the seed away from the die at the appropriate speed, a crystal corresponding to the shape of the growth control surfaces is formed.
Usually, the crystal seed is formed on one end of a generally elongated, relatively flat crystalline stem, the other end of which is adapted to be clamped to a crystal pulling mechanism. In the usual clamping arrangement the end of the stem to be clamped is formed with two spaced apart holes. A seed holder forming part of the crystal pulling mechanism includes two clamp members, one for each side of the stem. These clamp members are also formed with holes and these holes are located so as to be aligned with the holes in the stem. After aligning the holes suitable fasteners are used to clamp the seed between the clamp members.
In use the clamping arrangement described above suffers from various problems. Overtightening the fasteners can cause the stem to fracture or break so that the seed is no longer usuable. An additional problem results from the fact that the rate of thermal expansion of seed material is usually significantly greater than the rate of thermal expansion of the material from which the clamp members are made. If the seed material is aluminum oxide (Al.sub.2 O.sub.3) used in growing sapphire, the clamping material is generally molybdenum and the operating temperature is at least 2040.degree. C. The rate of thermal expansion of aluminum oxide is approximately two and a half times that of molybdenum so that the seed and the seed holder experience different expansion. This can induce thermal stresses in the stem that can cause the seed to fracture or break. Another problem can occur when the seed is lowered to its initial position relative to growth control surfaces. Overtravel of the seed can occur, that is, it can be lowered too far so that the seed touches the hot growth control surfaces. If the temperature is significantly high as is the case when growing sapphire, the growth control surfaces are in a relatively plastic state such that contact of these surfaces by the seed causes the seed to embed in and damage them.